TWI655299B - High-strength steel plate and manufacturing method thereof - Google Patents

Abstract

The present invention provides a high-strength steel sheet having bending workability and containing a large amount of Mn, which can be suitably used as a blank for automobile parts, and a method for manufacturing the same. A high-strength steel sheet with excellent bendability includes a center portion of the plate thickness and a single-sided or double-sided surface layer softening portion formed in the center portion of the plate thickness. The high-strength steel plate with excellent bendability is characterized in that: The average Mn concentration of the surface layer is greater than 4.0% by mass and less than 10.0% by mass. Each surface layer softened portion has a thickness of 0.1% to 30% of the plate thickness. The surface layer softened portion has a Mn concentration of 2.5% by mass or less. The recrystallization rate is 90% or more.

Description

High-strength steel plate and manufacturing method thereof

The present invention relates to a high-strength steel sheet with high Mn concentration and excellent bending properties, and a method for manufacturing the same.

BACKGROUND ART In order to achieve both weight reduction and safety of automobile bodies and parts, steel sheets used as the raw materials of these materials are continuously being developed with high strength. Generally, if the strength of a steel sheet is increased, the elongation will decrease and the formability of the steel sheet will be impaired. Therefore, in order to use a high-strength steel sheet as an automobile component, it is necessary to improve both the strength and formability of the opposite characteristics.

In order to improve the elongation, a so-called TRIP steel (for example, Patent Document 1) has been proposed as a steel that utilizes the deformation induced plasticity of residual Vostian iron (residual γ).

In addition, a steel with more than 4.0% Mn added has been proposed as a steel plate having a residual amount of Vastfield iron greater than the above-mentioned TRIP steel and having ductility exceeding the above-mentioned TRIP steel (for example, Non-Patent Document 1). Since the above-mentioned steel contains a large amount of Mn, the effect of reducing the weight of components using the same is also significant.

However, since the above-mentioned steel contains a large amount of Mn, Mn segregation at the time of solidification becomes apparent. In a structure with significant Mn segregation, a band-like structure with a hard structure tends to be formed in the Mn-concentrated region.

It is generally known that once a band-like structure is formed, in the forming accompanied by local deformation such as bending, localization of deformation is easy to occur and the deformation concentrated portion becomes the starting point of cracking, so the formability is significantly deteriorated.

Therefore, in order to realize a steel having excellent bending workability and containing a large amount of Mn, it is important to reduce Mn segregation.

For example, Patent Document 2 discloses a steel sheet having excellent formability. As shown in the example, it is a method of temporarily heating a cold-rolled and pickled steel sheet by using a steel sheet containing 20% or more of Asada's loose iron fraction. In the temperature range above 750 ° C, the Mn concentrated in the band structure is dispersed, and the thickness of the band structure is thinned and finely dispersed.

However, in the bending process of steel plates, a large tensile stress will be applied to the circumferential direction of the curved outer surface portion, and a large compressive stress will be applied to the curved inner surface portion, so the state of the surface layer will greatly affect the Bendability of high strength cold rolled steel sheet.

Therefore, the following hypothesis was established: by improving the Mn segregation of the surface layer, the tensile stress and compressive stress generated on the surface of the steel sheet during bending processing can be reduced, and the bendability can be improved.

Here, as a means for modifying the surface layer, it is conceivable to use a clad steel sheet.

Patent Document 3 discloses a method for manufacturing a clad steel sheet, which can reduce warpage and is made of one of a base metal steel, stainless steel, Ni, and an Ni alloy.

In addition, as a means for modifying the surface layer, the use of a cold spray method has also been proposed. Patent Document 4 discloses a cold-rolled steel sheet having a substrate portion containing Mn and a deposition layer (surface layer) having a low Mn concentration, and the deposition layer (surface layer) having a low Mn concentration is formed on at least the substrate portion by a cold spray method. One side.

Based on the above assumptions, the inventors have produced a clad steel sheet in which the average Mn concentration in the surface layer is lower than the average Mn concentration in the center layer, and the clad steel sheet has been examined for its bendability after cold rolling and annealing.

In addition, as described in several examples of Patent Document 4, a substrate portion having a high average Mn concentration is hot-rolled before forming a build-up layer (surface layer), and an average is formed on the substrate portion after hot-rolling by a cold spray method. The deposited layer (surface layer) having a low Mn concentration was examined for the bendability of the steel sheet after cold rolling and annealing. Further, as described in other examples of Patent Document 4, a cold rolled sheet having a high average Mn concentration is used as a substrate portion, and a cold spray method is used to form a stacked layer (surface layer) having a low average Mn concentration on the cold rolled sheet, and The bendability of the annealed steel sheet was also investigated.

However, it was found that even if the Mn segregation in the surface layer had been alleviated, the bendability could not be improved.

Prior Art Literature Patent Literature Patent Literature 1: Japanese Patent Laid-Open No. 5-59429 Patent Literature 2: Japanese Patent Laid-Open No. 2002-88447 Patent Literature 3: Japanese Patent Laid-Open No. 1-192404 Patent Literature 4: Japanese Patent Non-Patent Document No. 2015-193892

Non-Patent Document 1 Takagawa Furukawa and Matsumura, "Formation and Mechanical Properties of Remaining Vostian Iron in Low-Carbon Steels That Have Been Simply Heat-treated" ("Heat Treatment", Japan Heat Treatment Association, Heisei 9, Vol. 37 , No. 4), p. 204

SUMMARY OF THE INVENTION Problems to be Solved by the Invention An object of the present invention is to provide a steel sheet having bending workability and containing a large amount of Mn, which is capable of reliably solving the problems existing in the above-mentioned conventional technologies, and which is applicable as a blank for automobile parts and a method for manufacturing the same. .

Means for Solving the Problem The present inventors have actively conducted a review in order to solve the problem of the bendability of an ultra-high-strength steel sheet. First, the present inventors investigated the main reason why the bendability was not improved even when a clad steel sheet having a lower average Mn concentration in the surface layer than in the center layer was used.

As a result, it was found that the main reason for the deterioration of the bendability was: in order to improve the ductility of the center layer containing a large amount of Mn, the annealing temperature after cold rolling must be set to a low temperature, but on the other hand, in the low-temperature annealing, The recrystallization of the surface layer did not proceed sufficiently, resulting in a hard, non-recrystallized structure as a starting point for cracking.

In addition, the inventors have also pointed out that even if a cold-rolled steel sheet whose average Mn concentration of the build-up layer (surface layer) formed by the cold spraying method is reduced to be lower than the average Mn concentration of the substrate portion is used, the bendability cannot be improved. The main reasons were investigated.

As a result, it was found that if a substrate portion having a high average Mn concentration is hot-rolled before forming a build-up layer (surface layer), a cold spray method is used to form a low-average Mn concentration on the substrate portion after hot-rolling ( (Surface layer), in the case of a steel sheet produced by cold rolling and annealing in parallel, the recrystallization rate of the deposited layer (surface layer) is poor, so the bendability will not be improved.

It was also found that when a cold-rolled sheet with a high average Mn concentration was used as the substrate portion, and a cold-sprayed method was used to form a stacked layer (surface layer) with a low average Mn concentration on the cold-rolled sheet, and then annealing was performed to obtain a steel sheet, Since pores are generated in the stacked layer (surface layer), the flexibility is not improved. In addition, it was found that since the crystal grain size of the build-up layer (surface layer) is coarsened, sufficient bendability cannot be secured.

Therefore, the present inventors conducted a more detailed review. As a result, it was found that a steel plate having certain characteristics was welded to both sides of the base material, and hot-rolled and cold-rolled were produced under specific conditions to obtain a cold-rolled plate. By annealing the cold-rolled plate under specific conditions, It can maintain the ductility of the center layer and can best improve the bendability.

The mechanism of this effect is thought to be that the Mn segregation is suppressed due to the reduction of the Mn concentration in the surface layer of the multilayer steel sheet, and the Mn segregation has been sufficiently recrystallized, so that the localization of deformation in the curved surface is suppressed, and the surface layer ductility is suppressed The rupture occurred. In addition, it is also conceivable that the recrystallized grain size (ferrous grain iron grain size) of the surface layer becomes fine, so that the bendability is further improved.

The gist of the present invention obtained as described is as follows. (1) A high-strength steel plate comprising a center portion of the plate thickness and a single-sided or double-sided surface layer softening portion formed in the center portion of the plate thickness; The high-strength steel plate is characterized by: the average Mn concentration in the center portion of the plate thickness More than 4.0% by mass and less than 10.0% by mass, each surface layer softened portion has a thickness of 0.1% to 30% of the plate thickness, the average Mn concentration of the surface layer softened portion is 2.5% by mass or less, and the recrystallization rate of the surface layer of the surface layer softened portion is It is 90% or more, and the average crystal grain size of the recrystallized structure in the softened part of the surface layer is 0.1 μm or more and 40 μm or less. (2) The high-strength steel sheet according to the above (1), wherein the center portion of the aforementioned plate thickness is contained in mass%: C: more than 0.05% and less than 0.80%, Si: 0.001% and more and less than 3.50%, and Mn: more than 4.0% And less than 10.0%, P: 0.10% or less, S: 0.010% or less, sol.Al: 0.001% or more and less than 3.00%, and N: less than 0.050%, and the remainder is composed of iron and unavoidable impurities. (3) The high-strength steel sheet according to the above (2), wherein the center portion of the aforementioned plate thickness includes at least one element selected from the group consisting of: Cr: 0.01% or more and 2.00% or less, Mo: 0.01% or more and 2.00% or less, Cu: 0.01% or more and 2.00% or less, and Ni: 0.01% or more and 2.00% or less. (4) The high-strength steel sheet according to (2) or (3) above, wherein the center of the thickness of the aforementioned plate further includes at least one element selected from the group consisting of: Ti: 0.005% or more and 0.30% or less, Nb: 0.005% or more and 0.30% or less, V: 0.005% or more and 0.30% or less, and W: 0.005% or more and 0.30% or less. (5) The high-strength steel sheet according to any one of (2) to (4) above, wherein the center portion of the aforementioned plate thickness includes at least one element selected from the group consisting of: B: 0.0001% or more and 0.010% or less, Ca: 0.0001% or more and 0.010% or less, Mg: 0.0001% or more and 0.010% or less, Zr: 0.0001% or more and 0.010% or less, and REM: 0.0001% or more and 0.010% or less. (6) The high-strength steel sheet according to any one of the above (2) to (5), wherein the central part of the aforementioned plate thickness includes at least one element selected from the group consisting of: Sb: 0.0005% or more and 0.050% or less, Sn: 0.0005% or more and 0.050% or less, and Bi: 0.0005% or more and 0.050% or less. (7) The high-strength steel sheet according to any one of (2) to (6), wherein the amount of C in the softened portion of the surface layer is 0.9 times or less the amount of C in the center portion of the plate thickness. (8) The high-strength steel sheet according to any one of (3) to (7) above, wherein the sum of the amount of Cr and Mo in the softened portion of the surface layer is 0.9 of the sum of the amount of Cr and Mo in the center portion of the plate thickness. Times below. (9) The high-strength steel sheet according to any one of (3) to (8) above, wherein the sum of the amount of Cu and the amount of Ni in the softened portion of the surface layer is 0.9 of the sum of the amount of Cu and the amount of Ni in the center portion of the plate thickness. Times below. (10) The high-strength steel sheet according to any one of (4) to (9) above, wherein the sum of the amount of Ti and the amount of Nb in the softened portion of the surface layer is 0.9 of the sum of the amount of Ti and the amount of Nb in the center portion of the plate thickness Times below. (11) The high-strength steel sheet according to any one of (4) to (10) above, wherein the sum of the amount of V and W in the softened portion of the surface layer is 0.9 of the sum of the amount of V and W in the center portion of the plate thickness. Times below. (12) The high-strength steel sheet according to any one of (5) to (11), wherein the amount of B in the softened portion of the surface layer is 0.9 times or less the amount of B in the center portion of the plate thickness. (13) The high-strength steel sheet according to any one of (1) to (12) above, further comprising a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electro-galvanized layer on the surface of the surface layer softened portion. (14) A method for manufacturing a high-strength steel sheet, which is a method for manufacturing a high-strength steel sheet according to any one of (1) to (13) above; the method for manufacturing the high-strength steel sheet is characterized by including the following steps: One or both sides of the base material steel plate at the center of the plate thickness are laminated to form the steel plate for the surface softening portion of the surface softening portion to form a multilayer steel plate; heating the multilayer steel plate to a heating temperature of 1080 ° C to 1300 ° C, and Hot rolling is carried out at a starting rolling temperature of 800 ° C to 1000 ° C; and within 2 seconds after the finish rolling is finished, the multi-layer steel sheet after the hot rolling is cooled to above 500 ° C and 700 ° C The following; after cooling the multilayer steel sheet to a temperature of 500 ° C or more and 700 ° C or less, it is maintained for 3 seconds or more; after the multilayer steel sheet is maintained at a temperature of 500 ° C or more and 700 ° C or less for 3 seconds or more, Then, cold rolling is performed at a reduction ratio of 20% to 70%; and the multilayer steel sheet after the cold rolling is maintained at a temperature of 600 ° C or higher and 750 ° C or lower for 5 seconds or more, followed by cooling. (15) The method for manufacturing a high-strength steel sheet according to the above (14), wherein the multilayer steel sheet is wound at a coiling temperature of 600 ° C. or lower and held at a temperature of 500 ° C. or higher and 700 ° C. or lower for 3 seconds or longer. (16) The method for manufacturing a high-strength steel sheet according to the above (15), in which the multi-layer steel sheet after coiling is held at a temperature of 300 ° C or higher and 550 ° C or lower and tempered before the cold rolling.

Effects of the Invention According to the present invention, it is possible to provide a high-strength steel sheet having an excellent balance of strength and ductility, excellent bending characteristics, and a high Mn-containing concentration.

Embodiments of the Invention Embodiments of the invention will be described below. The present invention is not limited to the following embodiments.

1. Structure of Surface Softening Section The reason why the structure of the steel sheet of the present invention is specified in the above manner will be explained. In the following description, unless otherwise specified, "%" indicating the content of each element means mass%.

The average Mn concentration of the surface layer softened portion of the steel sheet of the present invention must be 2.5% by mass or less and the recrystallization rate must be 90% or more. The surface layer softened portion has a thickness of 0.1% to 30% of the plate thickness.

(The thickness of the softened portion of the surface layer is 0.1% to 30%) If the thickness of the softened portion of the surface layer is less than 0.1% of the plate thickness, the bendability cannot be sufficiently improved, and if it is more than 30%, the deterioration of tensile strength will become obvious. . The thickness of the surface softened portion is preferably 20% or less of the plate thickness, and more preferably 10% or less.

The "surface softening part" is determined as follows. First, the cross-section structure of the steel plate is exposed by the nitrate etchant, and the total thickness of the steel plate is calculated from the structure image obtained by observation with an optical microscope or a scanning electron microscope. The Vickers hardness is measured at the center of the plate thickness direction of the steel plate at a punching interval that does not interfere with each other in a direction perpendicular to the plate thickness direction. The Vickers hardness is measured at a pressure of 100 g for 5 points and the average of these is used. The value is taken as the average Vickers hardness at the center position in the thickness direction. Next, from the center of the plate thickness direction to the surface, the punching interval is set to a fixed interval of 5% of the total thickness of the steel plate, and the Vickers hardness test at 5 points is performed at the same position in the plate thickness direction as described above. When the average Vickers hardness at a position in the thickness direction is 0.6 times or less the average Vickers hardness at the center position in the thickness direction, the surface side softer than the position is defined as the surface layer softened portion. When printing at a 5% interval cannot obtain a value less than 0.6 times the average Vickers hardness and cannot define the surface softened portion, the interval between the two printed points on the surface is used to reduce the interval from the initial printing. Print at any shorter fixed interval to define the surface softening.

(The average Mn concentration in the softened portion of the surface layer is 2.5% by mass or less) Mn is an element that promotes the formation of a band-like structure. If the average Mn concentration in the surface layer softened portion is more than 2.5% by mass, a band-like structure is formed in the surface layer softened portion, and good bendability cannot be obtained. Therefore, the average Mn concentration in the surface softened portion is set to 2.5% by mass or less, preferably 2.0% by mass or less, and more preferably 1.5% by mass or less. Since the band-shaped structure becomes more difficult to form as the amount of Mn decreases, the lower limit of the average Mn concentration is not particularly limited. In addition, the "average Mn concentration in the surface softened portion" is measured at the center of the plate thickness direction of the tissue cross section along a line perpendicular to the plate thickness direction by EPMA at 50 μm intervals at 20 points, and the average value is calculated from the average value. Seeking.

(The recrystallization rate of the softened part of the surface layer is 90% or more.) If the recrystallization of the surface layer is insufficient and an unrecrystallized structure is present, it will become a starting point of cracking during bending due to poor ductility of the unrecrystallized structure. If the recrystallization rate of the surface layer softened portion is less than 90%, good bendability cannot be obtained, so the recrystallization rate of the surface layer softened portion is set to 90% or more. And it is preferably at least 95%.

(The average crystal grain size in the softened part of the surface layer is 0.1 μm or more and 40 μm or less) If the average crystal grain size of the recrystallized structure (ferrous iron) in the surface layer is coarsened, the unevenness of deformation is promoted during bending deformation, so that It becomes a main cause of deterioration of flexibility. Therefore, the average crystal grain size of the surface layer softened portion is set to 40 μm or less. The average crystal grain size of the softened portion of the surface layer is preferably 30 μm or less, and more preferably 25 μm or less. On the other hand, if the average crystal grain size becomes 0.1 μm or less, the ductility of the surface layer softened portion is significantly deteriorated. Therefore, the average crystal grain size of the surface layer softened portion is set to 0.1 μm or more. The average crystal grain size of the softened portion of the surface layer is preferably 0.5 μm or more, and more preferably 1 μm or more.

The "recrystallization rate" is determined as follows. In the recrystallization rate measurement test by the SEM / EBSD method, the surface of the steel plate is subjected to mirror polishing and colloidal polishing, so that the center position in the thickness direction of the surface softening portion defined by the above method becomes the measurement surface, and a field emission scanning method is used. An electron microscope (FE-SEM) and an OIM crystal orientation analysis device acquire crystal orientation data groups at intervals of 0.2 μm in a 100 μm square area of the measurement surface. An analysis software (TSL OIM Analysis) was used to analyze the crystal orientation data group, and the area where the Kernel Average Misorientation (KAM value) between the first measurement points was 1.0 ° or less was defined as the recrystallized structure, and the relative area of the area was calculated. The area ratio of the total area.

The average crystal grain size of the surface layer softened portion is determined as follows. In the average crystal particle size measurement test by the SEM / EBSD method, the crystal orientation data group obtained by the above method was analyzed by using an analysis software (TSL OIM Analysis), and the crystal orientation difference was calculated by the Area Fraction method to be 15 ° or more. The area surrounded by the crystal grain boundaries of the azimuth difference is defined as the particle size of one crystal grain, and the average particle size of the entire observation area is calculated.

2. Chemical composition of plate thickness center portion Next, the chemical composition of the plate thickness center portion (center layer) for obtaining the effect of the present invention will be described. In addition, as long as there is no special definition of "%" regarding element content, it means "mass%".

(C: more than 0.05% and less than 0.80%) C is an extremely important element in order to improve the strength of the steel and ensure the residual Vosted iron. In order to maintain sufficient strength and obtain a residual amount of iron in Wastfield, the C content must be greater than 0.05%. On the other hand, when excessive C is contained, the weldability of the steel sheet is impaired, so the upper limit of the C content is set to less than 0.80%. The C content is preferably in a range of 0.60% or less, and more preferably in a range of 0.50% or less.

(Si: 0.001% or more and less than 3.50%) Si is an effective element for strengthening tempered Asada iron and homogenizing the structure to improve workability. In addition, Si also has the effect of suppressing the precipitation of cis-carbon iron and promoting the retention of Vostian iron. In order to obtain the above effect, it is necessary to have a Si content of 0.001% or more. On the other hand, when excessive Si is contained, the low temperature toughness of the steel sheet is impaired, so the upper limit of the Si content is set to less than 3.50%. The lower limit of Si is preferably 0.01%, preferably 0.30%, and more preferably 0.50%. By setting the lower limit of the Si content within the above range, the uniform elongation characteristics of the steel sheet can be further improved. The upper limit of the Si content is preferably 3.00%, and more preferably 2.50%.

(Mn: more than 4.00% and less than 10.0%) Mn is an element that stabilizes Vosstian iron and improves hardenability. In addition, in the steel sheet of the present invention, Mn is distributed in the Vosstian iron to stabilize the Vosstian iron. In order to stabilize Vosstian iron at room temperature, it is necessary to have Mn greater than 4.00%. On the other hand, if the steel sheet contains excessive Mn, the low-temperature toughness is impaired. Therefore, the upper limit of the Mn content is set to less than 10.0%. The lower limit of the Mn content is preferably 4.30%, and more preferably 4.80%. The upper limit of the Mn content is preferably 8.00%, and more preferably 7.50%. By setting the lower limit value and the upper limit value of the Mn content within the above range, the ductility can be obtained more stably. The "average Mn concentration at the center of the plate thickness" refers to the center of the plate thickness direction at the center of the plate thickness of the tissue cross section, along the line perpendicular to the plate thickness direction, and measured the Mn concentration at 20 points by EPMA at 50 μm intervals. And calculate from its average.

(sol.Al: 0.001% or more and less than 3.00%) Al is a deoxidizer and must contain 0.001% or more. In addition, since Al expands the two-phase temperature range during annealing, it also has the effect of improving material stability. Although the effect becomes larger as the content of Al increases, if excessive Al is contained, surface properties, paintability, and weldability are deteriorated. Therefore, the upper limit of sol.Al is set to less than 3.00%. The lower limit of the sol.Al content is preferably 0.005%, preferably 0.01%, and more preferably 0.02%. And the upper limit of the sol.Al content should be 2.50%, more preferably 1.80%. By setting the lower limit value and the upper limit value of the sol.Al content within the above range, the balance between the deoxidizing effect and the effect of improving the stability of the material and the surface properties, paintability and weldability will become better.

(P: 0.10% or less) P is an impurity. If the steel sheet contains excessive P, the toughness and weldability will be impaired. Therefore, the upper limit of the P content is set to 0.10% or less. The upper limit of the P content is preferably 0.050%, preferably 0.030%, and more preferably 0.020%. Since the steel sheet of this embodiment does not necessarily have P, the lower limit of the P content is 0.000%. The lower limit of the P content may be greater than 0.000% or 0.001%, but the smaller the P content, the better.

(S: 0.010% or less) S is an impure substance. If the steel sheet contains excessive S, hot-rolling will generate elongated MnS, resulting in deterioration of bendability and formability such as hole expandability. Therefore, the upper limit of the S content is set to 0.010% or less. The upper limit of the S content is preferably 0.007%, and more preferably 0.003%. Since the steel sheet of this embodiment does not necessarily need to have S, the lower limit of the S content is 0.000%. The lower limit of the S content can be set to greater than 0.000% or 0.0001%, but the smaller the S content, the better.

(N: less than 0.050%) N is an impurity, and if the steel sheet contains N in an amount of 0.050% or more, the toughness will be deteriorated. Therefore, the upper limit of the N content is set to less than 0.050%. The upper limit of the N content is preferably 0.010%, and more preferably 0.006%. Since the steel sheet of this embodiment does not necessarily have N, the lower limit of the N content is 0.000%. The lower limit of the N content may be set to be greater than 0.000% or may be set to 0.0003%, but the smaller the N content, the better.

(Cr: 0.01% or more and 2.00% or less, Mo: 0.01% or more and 2.00% or less, Cu: 0.01% or more and 2.00% or less and Ni: 0.01% or more and 2.00% or less) For the steel sheet of this embodiment, Cr , Mo, Cu, and Ni are not essential elements, respectively. However, since Cr, Mo, Cu, and Ni are elements which can increase the strength of the steel sheet, they may also be contained. In order to obtain the effect of improving the strength of the steel sheet, the steel sheet may also contain at least 0.01% of one or more elements selected from the group consisting of Cr, Mo, Cu, and Ni. However, if the steel sheet contains excessive amounts of these elements, it is easy to generate surface flaws during hot rolling, and the strength of the hot rolled steel sheet may become too high and the cold rolling ductility may be reduced. Therefore, the upper limit of the content of each of one or more elements selected from the group consisting of Cr, Mo, Cu, and Ni is set to 2.00%.

(Ti: 0.005% or more and 0.30% or less, Nb: 0.005% or more and 0.30% or less, V: 0.005% or more and 0.30% or less and W: 0.005% or more and 0.30% or less) For the steel sheet of this embodiment, Ti , Nb, V, and W are not essential elements. However, Ti, Nb, V, and W are elements that can form fine carbides, nitrides, or carbonitrides, and are therefore effective for improving the strength of steel sheets. Therefore, the steel sheet may contain one or more elements selected from the group consisting of Ti, Nb, V, and W. In order to obtain the effect of improving the strength of the steel sheet, the lower limit of the respective content of one or more elements selected from the group consisting of Ti, Nb, V, and W should be set to 0.005%. On the other hand, if these elements are contained excessively, the strength of the hot-rolled steel sheet may increase excessively and the cold-rolled ductility may decrease. Therefore, the upper limit of the content of each of one or more elements selected from the group consisting of Ti, Nb, V, and W is set to 0.30%.

(B: 0.0001% or more and 0.010% or less, Ca: 0.0001% or more and 0.010% or less, Mg: 0.0001% or more and 0.010% or less, Zr: 0.0001% or more and 0.010% or less and REM: 0.0001% or more and 0.010% or less ) B, Ca, Mg, Zr and REM are not essential elements. However, B, Ca, Mg, Zr and REM will improve the local ductility and hole expandability of the steel sheet. In order to obtain this effect, the lower limit of one or two or more elements selected from the group consisting of B, Ca, Mg, Zr, and REM should preferably be 0.0001%, preferably 0.001%. . However, an excessive amount of these elements will deteriorate the workability of the steel sheet. Therefore, the upper limit of the content of each of these elements should be set to 0.010% or less, and it is selected from the group consisting of B, Ca, Mg, Zr, and REM. The total content of one or more elements should preferably be 0.030% or less.

(Sb: 0.0005% or more and 0.050% or less, Sn: 0.0005% or more and 0.050% or less and Bi: 0.0005% or more and 0.050% or less) Sb, Sn, and Bi are not essential elements. However, Sb, Sn, and Bi inhibit the diffusion of oxidizable elements such as Mn, Si, and / or Al to the surface of the steel sheet to form oxides, and can improve the surface properties and plating properties of the steel sheet. In order to obtain this effect, the lower limit of the content of each of the one or two or more elements selected from the group consisting of Sb, Sn, and Bi is preferably set to 0.0005%, and preferably 0.001%. On the other hand, if the content of each of these elements is greater than 0.050%, the effect is saturated, so the upper limit of the content of each of these elements is set to 0.050%.

3. Chemical composition of the surface softened portion As far as elements other than Mn are concerned, the steel sheet of the present invention may have different chemical compositions in the surface softened portion and the thickness center portion. In this case, the preferable chemical composition of the surface softened portion is as follows.

(C: 0.9 times or less the amount of C in the center of the plate thickness) C is an element that can increase the strength of the steel sheet and is added to increase the strength of the high-strength steel sheet. The amount of C in the softened portion of the surface layer should be 0.9 times or less the amount of C in the center portion of the plate thickness. The reason is to reduce the hardness of the softened portion of the surface layer to be lower than that of the center portion of the plate thickness. If it is more than 0.9 times, excellent bendability may not be obtained. The amount of C in the softened portion of the surface layer is preferably 0.7 times or less, more preferably 0.5 times or less, and most preferably 0.3 times or less. Since the preferred C content in the central portion of the plate thickness is less than 0.80%, the preferred C content in the surface softened portion is less than 0.72%. It should be less than 0.5%, more preferably less than 0.3%, and most preferably less than 0.1%. The lower limit of the amount of C is not particularly specified. When extremely low C steel for industrial use is used, the actual lower limit is about 0.001%. However, from the viewpoint of the so-called solid solution C amount, Interstitial Free steel that completely excludes solid solution C can also be used by using Ti or Nb.

(Si: 0.001% or more and less than 3.50%) The Si-based ferrous iron stabilizing element can increase the Ac3 abnormality point, so that a large amount of ferrous iron can be formed at a wide range of annealing temperatures. Add to. In order to obtain the effect, the amount of Si must be set to 0.001% or more. However, the addition of 3.50% or more deteriorates the surface properties of the steel sheet, so it is set to less than 3.50%.

(P: 0.10% or less) P embrittles the welded portion. If it is more than 0.10%, the embrittlement of the welded portion becomes significant, so the appropriate range is limited to 0.10% or less. Although the lower limit of the P content is not specified, if it is set to less than 0.001%, it is economically disadvantageous.

(S: 0.010% or less) S adversely affects weldability and manufacturability during casting and hot rolling. From this, the upper limit 设 is set to 0.010% or less. Although the lower limit of the S content is not specified, if it is set to less than 0.0001%, it is economically disadvantageous.

(sol.Al: 0.001% or more and less than 3.00%) Al can function as a deoxidizing agent, and it is suitable to be added in the deoxidizing step. In order to obtain the effect, the sol.Al content must be 0.001% or more. On the other hand, when the sol.Al content is 3.00% or more, the risk of cracking of the steel billet during continuous casting increases, so it is set to less than 3.00%.

(N: 0.050% or less) N forms coarse nitrides and deteriorates bendability. Therefore, the amount of N must be suppressed. The reason is that if N is greater than 0.050%, the tendency will be significant, so the N content range is set to 0.050% or less. In addition, N should cause fewer pores during welding. The lower limit 値 of the N content is not particularly limited so that the effects of the present invention can be exerted, but if the N content is set to less than 0.0005%, the manufacturing cost will increase significantly.

The surface softening section preferably has the following composition: Contained in mass%: C: less than 0.72%, Si: 0.001% or more and less than 3.50%, Mn: 2.5% or less, P: 0.10% or less, S: 0.010% or less, sol. Al: 0.001% or more and less than 3.00%, and N: less than 0.050%, and the remainder is composed of iron and unavoidable impurities. The surface layer softened portion may further contain the following components.

(Cr: 0.01% or more and 2.00% or less, Mo: 0.01% or more and 2.00% or less, Cu: 0.01% or more and 2.00% or less and Ni: 0.01% or more and 2.00% or less) Cr, Mo, Cu, and Ni are acceptable It can also contain elements that increase the strength of the steel sheet. The steel sheet may contain 0.01% or more of one or more elements selected from the group consisting of Cr, Mo, Cu, and Ni, respectively. However, if the steel sheet contains excessive amounts of these elements, the strength of the steel sheet may become too high, and it may become easy to generate surface flaws during rolling delay. Therefore, the upper limit of the content of each of one or more elements selected from the group consisting of Cr, Mo, Cu, and Ni is set to 2.00%.

The total amount of Cr and Mo in the surface softened portion is preferably 0.9 times or less of the total amount of Cr and Mo in the center portion of the plate thickness. If the sum of the amount of Cr and Mo that stabilizes carbides is greater than 0.9 times the amount of Cr and Mo in the center of the plate thickness, coarse carbides tend to remain and cause surface deterioration. It is preferably 0.7 times or less, more preferably 0.5 times or less, and most preferably 0.3 times or less.

The sum of the amount of Cu and the amount of Ni in the surface softened portion should preferably be 0.9 times or less the sum of the amount of Cu and the amount of Ni in the center portion of the plate thickness. If the sum of the amount of Cu and Ni that can improve the hardenability is greater than 0.9 times the amount of Cu and Ni in the center of the plate thickness, a low-temperature deformed structure is likely to occur, which may cause the deterioration of bendability. It is preferably 0.7 times or less, more preferably 0.5 times or less, and most preferably 0.3 times or less.

(Ti: 0.005% or more and 0.30% or less, Nb: 0.005% or more and 0.30% or less, V: 0.005% or more and 0.30% or less and W: 0.005% or more and 0.30% or less) Ti, Nb, V, and W are acceptable An element that generates fine carbides, nitrides, or carbonitrides is effective for improving the strength of steel sheets. Therefore, the steel sheet may contain one or more elements selected from the group consisting of Ti, Nb, V, and W. In order to obtain the effect of improving the strength of the steel sheet, the lower limit value of each content of one or two or more elements selected from the group consisting of Ti, Nb, V, and W should be 0.005%. On the other hand, if these elements are contained excessively, the strength of the hot-rolled steel sheet may be excessively increased, and there is a possibility of cracking during the cold rolling delay. Therefore, the upper limit of the content of each of one or more elements selected from the group consisting of Ti, Nb, V, and W is set to 0.30%.

The total amount of Ti and Nb in the surface softened portion should preferably be 0.9 times or less the total amount of Ti and Nb in the center portion of the plate thickness. If the sum of the amount of Ti and the amount of Nb is greater than 0.9 times the amount of Ti and the amount of Nb in the central portion of the plate thickness, the surface layer is likely to be hardened, which may cause the deterioration of bendability. It is preferably 0.7 times or less, more preferably 0.5 times or less, and most preferably 0.3 times or less.

The total amount of W and V in the surface softened portion should preferably be 0.9 times or less the total of the amount of W and V in the center portion of the plate thickness. Since W and V are elements that are liable to form carbides, if the sum of the amount of W and V is greater than 0.9 times the amount of W and V in the center of the plate thickness, coarse carbides will be formed on the surface layer, which will cause bendability. The main cause of deterioration. It is preferably 0.7 times or less, more preferably 0.5 times or less, and most preferably 0.3 times or less.

(B: 0.0001% or more and 0.010% or less, Ca: 0.0001% or more and 0.010% or less, Mg: 0.0001% or more and 0.010% or less, Zr: 0.0001% or more and 0.010% or less and REM: 0.0001% or more and 0.010% or less ) B, Ca, Mg, Zr and REM will improve the local ductility and hole expansion of the steel plate. In order to obtain this effect, the lower limit of one or two or more elements selected from the group consisting of B, Ca, Mg, Zr, and REM should preferably be 0.0001%, preferably 0.001%. . On the other hand, if the amounts of B, Ca, Mg, Zr, and REM are greater than 0.9 times the center of the plate thickness, the surface layer may be excessively hardened and the surface properties may be deteriorated. Therefore, the upper limit of one or two or more elements selected from the group consisting of B, Ca, Mg, Zr, and REM is set to 0.009%, and more preferably 0.006% or less.

The amount of B in the surface softened portion is preferably 0.9 times or less the amount of B in the center portion of the plate thickness. B is an element that is liable to form a low-temperature metamorphic phase. Therefore, if the amount of B is greater than 0.9 times the amount of B in the center portion of the plate thickness, the surface layer becomes hard, which may cause failure to obtain excellent bendability. It is preferably 0.7 times or less, more preferably 0.5 times or less, and most preferably 0.3 times or less. These lower limits are not required.

4. Structure of steel plate The structure of the steel plate in this embodiment will be described.

The tissue at the center of the plate thickness should have a structure composed of ferrous grain iron, Asada loose iron or toughened iron, and residual Vostian iron, and preferably has a structure consisting of fat iron, tempered Asada iron loosened or toughened Iron, hardened Asada loose iron, and residual Vosted iron.

The structure of the surface softening portion should preferably have a structure consisting essentially of ferrous iron.

The porosity of the softened portion of the surface layer is small, and is preferably 1% or less in terms of area ratio, and more preferably 0%.

5. Mechanical characteristics of the steel sheet This section describes the mechanical characteristics of the steel sheet of this embodiment.

The tensile strength of the steel sheet in this embodiment is preferably 780 MPa or more, and more preferably 1180 MPa or more. This is because when a steel sheet is used as a raw material for automobiles, the thickness is reduced by increasing strength to contribute to weight reduction. In addition, in order to supply the steel sheet of this embodiment to press forming, it is preferable to have excellent uniform elongation (uEL). TS × uEL is preferably 12000 MPa ·% or more, and more preferably 14000 MPa ·% or more.

Regarding bendability, when performing a V-bend test in accordance with JIS Z2248 for a steel type having a strength of 780 MPa or more and less than 1180 MPa, the direction perpendicular to the rolling direction is taken as the long side direction (the bending ridge line is consistent with the rolling direction). The critical bending radius R is preferably 1.0 mm or less, and preferably 0.8 mm or less. For steel types with a strength of 1180 MPa or more, the critical bending radius of the above-mentioned V-bending test should be 2.0 mm or less, and preferably 1.5 mm or less.

6. Manufacturing method Next, a manufacturing method of the steel plate according to this embodiment will be described. The following description is intended only to illustrate the method for producing the high-strength steel sheet of the present invention, and is not intended to limit the high-strength steel sheet of the present invention to a multi-layer steel sheet formed by laminating two steel sheets as described below. For example, instead of the lamination method described below, a surface layer softened portion may be formed on a base material steel plate by a cold spray method.

The steel sheet of this embodiment is produced by hot-rolling a multi-layer steel sheet, cooling it immediately after rolling and keeping it at a high temperature, and then pickling the cooled steel sheet, followed by cold-rolling and annealing, to produce the multi-layer steel sheet. The base material steel plate having the above-mentioned chemical composition and constituting the center portion of the plate thickness is formed by laminating an average Mn concentration of 2.5% by mass or less and forming a multilayer steel plate for the steel sheet for the surface layer softening portion, and welding the surroundings.

(Formation of multi-layer steel plate: On one or both sides of the base material steel plate after degreasing the surface constituting the center portion of the plate thickness, a steel plate satisfying the chemical composition of the surface softening portion is laminated and welded around it.) On the base material steel sheet with the chemical composition in the center part, a steel sheet that satisfies the chemical composition in the surface softening part is laminated on the surface and welded around to form a multilayer steel sheet. As long as these steel plates satisfy the above-mentioned chemical composition, they may be obtained by any manufacturing method.

(Heating temperature of multilayer steel plate: 1080 ° C or more and 1300 ° C or less) If the heating temperature before hot rolling is less than 1080 ° C, the deformation resistance during hot working becomes high, which makes the operation difficult. On the other hand, if the heating temperature is higher than 1300 ° C, the yield will decrease due to scale wear. Therefore, the heating temperature is set to be 1080 ° C or more and 1300 ° C or less. The time during which the temperature is maintained in the temperature range from 1080 ° C to 1300 ° C before hot rolling is not particularly limited, but in order to improve the hole expandability, it is preferably set to 30 minutes or more, and more preferably 1 hour or more. In addition, in order to suppress excessive scale wear, it should be set to 10 hours or less, and more preferably 5 hours or less. When direct rolling or direct rolling is performed, the rolling may be performed while maintaining the temperature range as described above. In this specification, the temperature is the temperature measured at the center of the surface of the steel sheet.

(Finishing rolling start temperature: 800 ° C or higher and 1000 ° C or lower) The finishing rolling start temperature is preferably set to 800 ° C or higher and 1000 ° C or lower. By setting the finishing rolling start temperature to 800 ° C or higher, the deformation resistance of the rolling delay can be made small. On the other hand, by setting the finishing rolling start temperature to 1000 ° C. or lower, deterioration of the surface properties of the steel sheet due to grain boundary oxidation can be suppressed.

(Cooling after rolling: cooled to 500 ° C or higher and 700 ° C or lower within 2 seconds) Within 2 seconds after finishing rolling, cooled to 500 ° C or higher and 700 ° C or lower. This is a very important condition in the present invention. By making the old γ grains in the softened part of the surface layer fine and uniformly and finely forming the iron particles of the fertilizer particles that will be generated during cooling, the surface layer can be made in the subsequent annealing step. The softened portion was sufficiently recrystallized.

If the time from cooling to 500 ° C to 700 ° C is greater than 2 seconds from the end of the finishing rolling, the grain size of the old Vostian iron will become coarse. In the subsequent annealing step, the surface softening part will not be affected. Fully recrystallized. Therefore, the time from the completion of finishing rolling to cooling to 500 ° C or higher and 700 ° C or lower is set to within 2 seconds. It is preferably within 1.8 seconds, and preferably within 1.5 seconds. The shorter the time until cooling, the finer the old γ particle size becomes and it becomes easier to recrystallize, so the lower limit is not set, but 0.1 second is the substantial lower limit due to the limitation of the manufacturing steps.

As long as the cooling rate satisfies the above conditions, any speed may be used, and the faster the cooling rate, the easier it is to obtain the fine granulation effect of the old γ particle size. Therefore, the cooling rate is preferably 20 ° C / s or more, and more preferably 50 ° C / s or more.

If the cooling stop temperature after rolling is lower than 500 ° C, a part of the surface layer softened portion will become a low-temperature metamorphic structure. If there are many microstructures of ferrous iron and low-temperature metamorphic structures, deformation will be introduced unevenly during the cold rolling delay, so recrystallization will not occur uniformly, and unrecrystallized structures will easily remain. If the cooling stop temperature is 700 ° C. or higher, the deformation of the ferrite grains in the surface softening portion will be delayed. Therefore, sufficient strain cannot be accumulated in the surface softening portion in the subsequent cold rolling step. Therefore, the cooling stop temperature is set to 500 ° C or higher and 700 ° C or lower.

(Holding time after cooling to a temperature of 500 ° C or higher and 700 ° C or lower: 3 seconds or more) If the holding time of a temperature of 500 ° C or higher and 700 ° C or lower is less than 3 seconds, the ferrous iron in the surface softening portion will not be sufficient generate. The holding time is preferably 5 seconds or more, and more preferably 10 seconds or more.

(Rewinding temperature: 600 ° C or lower) It should be coiled at a coiling temperature of 600 ° C or lower. When coiling is performed at a coiling temperature of 600 ° C or lower, a low-temperature abnormal phase will be easily formed at the center of the plate thickness, and the amount of strain distribution to the surface layer will increase in the cold rolling step after coiling. The surface softened portion becomes easy to recrystallize, and it becomes easy to make the crystal grain size finer. In addition, by setting the winding temperature to 600 ° C. or lower, the scale can be more easily removed during pickling after winding. The winding temperature is preferably 500 ° C or lower, and more preferably 400 ° C or lower.

In order to suppress the delayed cracking of cold rolling, the hot-rolled sheet can also be tempered at a temperature above 300 ° C and below 600 ° C after cooling to room temperature.

(Cold rolling reduction: 20% or more and 70% or less) Hot-rolled steel sheets are cold-rolled after being pickled by a conventional method. If the cold rolling reduction ratio is less than 20%, sufficient strain cannot be introduced into the surface softened portion, and the surface softened portion will not be sufficiently recrystallized in the subsequent annealing step. On the other hand, if the rolling reduction rate of the cold rolling is greater than 70%, the steel sheet may be broken during rolling. Therefore, the rolling reduction rate of the cold rolling is set to be 20% or more and 70% or less.

(Annealed heat treatment after cold rolling: Hold the rolled multilayer steel plate at a temperature of 600 ° C or higher and 750 ° C or lower for 5 seconds or more, and then cool to room temperature.) Heat the cold rolled multilayer steel plate to 600 ° C. Annealed at a temperature above 750 ° C. If the heating and holding temperature is lower than 600 ° C, not only the surface softened portion will not be sufficiently recrystallized, but also the cis-carbon iron in the center portion of the plate thickness will not be fully melted, resulting in that a stable residual γ fraction cannot be obtained. If it is higher than 750 ° C., it becomes difficult to generate fertilizer iron at the center of the plate thickness. Therefore, the heating and holding temperature is set to 600 ° C or higher and 750 ° C or lower.

If the holding time is less than 5 seconds, the non-recrystallized structure in the softened portion of the surface layer will not be sufficiently recrystallized. In order to completely remove the non-recrystallized structure, the annealing time is preferably set to 10 seconds or more, and more preferably 15 seconds or more. From the viewpoint of productivity, the annealing time should preferably be 3600 seconds or less.

In order to generate a low-temperature metamorphic structure in the center portion of the plate thickness, the cooling stop temperature after the heating and maintaining is preferably 550 ° C or lower, preferably 300 ° C or lower, and most preferably 100 ° C or lower.

After the above cooling, in order to soften the low-temperature metamorphic structure and stabilize the residual Vosted iron, tempering can also be performed at a temperature of 300 ° C or higher and 550 ° C or lower.

When hot-dip galvanizing is performed on the surface of a steel sheet to manufacture a hot-dip galvanized steel sheet, the cooling after annealing at a temperature of 600 ° C to 750 ° C is stopped in a temperature range of 430 to 500 ° C, and then the cold The rolled steel sheet is immersed in a molten zinc plating bath and subjected to a hot-dip galvanizing treatment. The conditions of the plating bath may be set within a general range. After the plating process, it can be cooled to room temperature.

When alloyed hot-dip galvanizing is performed on the surface of a steel sheet to produce an alloyed hot-dip galvanized steel sheet, the steel sheet is melted at a temperature of 450 to 620 ° C after being subjected to a hot-dip galvanizing treatment on the steel sheet before being cooled to room temperature. Galvanized alloying. The alloying treatment conditions may be set within a general range.

By manufacturing the steel sheet as described above, the steel sheet of this embodiment can be obtained. Examples

The steel sheet of the present invention will be described more specifically with reference to examples. However, the following examples are examples of the steel sheet of the present invention, and the steel sheet of the present invention is not limited to the following examples.

1. Steel sheet for manufacturing evaluation The manufacturing method of the multi-layer steel sheet described in Table 2 is a sample of the cladding method, and was produced by the following method. After continuously grinding the surface of a 20-mm-thick continuous cast steel billet (steel plate at the center of the plate thickness) having a chemical composition shown in Table 1 to remove surface oxides, arc welding was applied to the single-sided or double-area layer. Steel sheet for chemical composition (surface softening part). The heating temperature, the holding time before hot rolling, the finishing rolling start temperature, the cooling completion time, the cooling stop temperature, the cooling holding time and the coiling temperature shown in Table 2 are used to heat and heat the obtained product. Post-holding, hot-rolling, cooling, holding after cooling, and coiling into coils to produce laminated hot-rolled steel sheets. Then, pickling was performed by a conventional method, and tempering, cold rolling, and annealing were performed at the tempering temperature, cold rolling reduction rate, annealing temperature, and annealing time shown in Table 2, and the mixture was cooled to room temperature.

The manufacturing method of the multi-layered steel sheet described in Table 2 is a sample of the cold spray method, and it was manufactured by the following method.

For the case where the substrate is a hot-rolled sheet, a 20-mm-thick continuous cast steel slab (steel plate at the center of the sheet thickness) having a chemical composition shown in Table 1 is used. Pre-holding time, finishing rolling start temperature, cooling completion time, cooling stop temperature, holding time after cooling and coiling temperature, heating, holding after heating, hot rolling, cooling, holding after cooling and coiling into coils The hot-rolled sheet is prepared from the material, and the surface oxide is removed on the ground surface, and then a single layer or a double-sided layer is formed on the surface layer by a cold spray method to form a stacked layer (surface layer) to produce a steel plate. Then, the tempering, cold rolling, and annealing were performed at the tempering temperature, cold rolling reduction, annealing temperature, and annealing time shown in Table 2, and then cooled to room temperature.

On the other hand, if the substrate is a cold-rolled sheet, the hot-rolled sheet produced by the above method is pickled by a conventional method, and is tempered at the tempering temperature and the cold rolling reduction rate shown in Table 2. Fire and cold rolling are used to produce cold-rolled sheets, and then a cold spray method is used to form a build-up layer (surface layer) on at least one side of the cold-rolled sheet, and annealing is performed at the annealing temperature and annealing time shown in Table 2 to cool to room temperature. temperature.

The iron-based particles used in the cold spray method are those having components and particle sizes shown in Table 4. The particles used in the cold spraying method are those which are adjusted to a predetermined particle diameter by repeatedly pulverizing and grading with a sieve according to the prior art. In addition, nitrogen is used as the working gas. The working gas is heated to 700 ° C. with a heater, iron-based particles are supplied from the particle supply device and mixed, and then the substrate is sprayed on the substrate with a spray nozzle to obtain a multilayer steel plate. The working pressure is fixed at 3 MPa. In addition, the scanning speed of the nozzle is adjusted by mechanical control.

[Table 1-1] [Table 1-2]

[table 2-1] [Table 2-2]

For some annealed cold-rolled steel sheets, after the final annealing, the cooling after annealing is stopped at 460 ° C, and the cold-rolled steel sheets are immersed in a molten zinc plating bath at 460 ° C for 2 hours to perform hot-dip zinc plating deal with. The conditions of the plating bath are the same as in the past. When the alloying treatment described later is not performed, after holding at 460 ° C, it is cooled to room temperature at an average cooling rate of 10 ° C / sec.

For a part of the annealed cold-rolled steel sheet, after being subjected to the hot-dip galvanizing treatment, the alloying treatment is continued without cooling to room temperature. It was heated to 520 ° C. and maintained at 520 ° C. for 5 seconds for alloying treatment, and then cooled to room temperature at an average cooling rate of 10 ° C./second.

The annealed cold-rolled steel sheet produced as described above was quenched and tempered at an elongation of 0.1%, and various steel sheets for evaluation were prepared.

2. Evaluation method The obtained annealed cold-rolled steel sheet is subjected to: plate thickness measurement, microstructure observation, porosity measurement of surface softened part, Vickers hardness test, recrystallization rate measurement test by SEM / EBSD method, surface softened part Measurement of average crystal grain size, tensile test, uniform extension test, and V-bend test.

The method of observing the structure at the center of the plate thickness is performed as follows. The surface of the steel plate was mirror-polished and colloid-polished to make the center of the plate thickness the measurement surface. Using a field emission scanning electron microscope (FE-SEM) and an OIM crystal orientation analysis device, the measurement surface was 100 μm square and the area was 0.2 μm. The crystal orientation data group was acquired at intervals. The crystal orientation data group obtained by analysis by the analysis software (TSL OIM Analysis), and the organization was classified. In Phase-MAP, a region determined as a Vosstian iron phase is identified as a residual Vosstian iron. Regarding the organization in the Phase-MAP area that is judged to be other than the Vostian iron phase, including tempered Asada loose iron, toughened iron, Asada loose iron maintained in the quenched state, and the fertile iron phase of the fertile iron, the basis is 3000 The secondary electron image obtained by double magnification observation is further discriminated as follows. Among the ferrous grain iron phases, among those having a lower structure in the crystal grains, the structure containing Xueming carbon iron inside was judged as tempered Asada loose iron or toughened iron. Among the ferrous phase, among those having a lower structure in the crystal grains, the structure that does not contain Schiff carbon iron in the interior is judged to be as hardened as Mata San. The area of the ferrous iron phase that does not contain the lower structure in the crystal grains is identified as the ferrous iron.

Observe the structure in the above method, and the structure of the central part of the plate thickness will be classified as fertile iron, tempered Asada iron or toughened iron, Asada loose iron maintained in the quenched state, and residual Vostian iron.

The structure observation of the surface layer softened portion was performed in the same manner as the structure observation of the center portion of the plate thickness, except that the surface layer softened portion was used as the measurement surface. The structure of the surface softening part is essentially fertilized iron.

The porosity of the surface softened portion is determined by identifying the porosity of the polished surface, and calculating the area ratio by image processing. The porosity of the softened part of the surface layer was observed with a scanning electron microscope at a magnification of 1000 times, and pores with a diameter of 0.01 μm or more were detected by image processing, and the total area ratio was calculated.

As described above, the Vickers hardness test is performed in order to define the softened portion of the surface layer. First, the cross-section structure of the steel plate is exposed by the nitrate etchant, and the total thickness of the steel plate is calculated from the structure image obtained by observation with an optical microscope or a scanning electron microscope. The Vickers hardness is measured at the center of the plate thickness direction of the steel plate at a punching interval that does not interfere with each other in a direction perpendicular to the plate thickness direction. The Vickers hardness is measured at a pressure of 100 g for 5 points and the average of these is used. The value is taken as the average Vickers hardness at the center position in the thickness direction. Next, from the center of the plate thickness direction to the surface, the print interval was set to a fixed interval of 5% of the total thickness of the steel plate, and a Vickers hardness test at 5 points was performed in the same position as in the plate thickness direction. When the average Vickers hardness at a position in the thickness direction is 0.6 times or less the average Vickers hardness at the center position in the thickness direction, the surface side softer than the position is defined as the surface layer softened portion. When printing at a 5% interval cannot obtain a value less than 0.6 times the average Vickers hardness and cannot define the surface softened portion, the interval between the two printed points on the surface is used to reduce the interval from the initial printing. A shorter arbitrary fixed interval is used for printing, and the surface softening part is defined. The ratio of the thickness of the steel plate for the surface layer softened portion to the plate thickness of the steel plate for the center portion of the plate thickness is shown in "Ratio (%) of the surface layer softened portion (one side)" in Table 3.

In the recrystallization rate measurement test by the SEM / EBSD method, the surface of the steel plate was subjected to mirror polishing and colloidal polishing, so that the center position of the surface softening portion defined according to the above method was the measurement surface, and a field emission scanning electron microscope ( FE-SEM) and OIM crystal orientation analysis device, obtained crystal orientation data groups at intervals of 0.2 μm in a 100 μm square area of the measurement surface. An analysis software (TSL OIM Analysis) was used to analyze the crystal orientation data group, and the area where the Kernel Average Misorientation (KAM value) between the first measurement points was 1.0 ° or less was defined as the recrystallized structure, and the relative area of the area was calculated. The area ratio of the total area.

The average crystal grain size of the surface layer softened portion was measured as follows. In the average crystal particle size measurement test by the SEM / EBSD method, the crystal orientation data group obtained by the above method was analyzed by using an analysis software (TSL OIM Analysis), and the crystal orientation difference was calculated by the Area Fraction method to be 15 ° or more. The area surrounded by the crystalline grain boundaries of the azimuth difference is defined as the particle size of one crystal grain, and the average particle size of the entire observation area is calculated.

The long axis was taken in a direction perpendicular to the rolling direction of the steel sheet, and a JIS No. 5 test piece was taken, and then the tensile strength (TS) and uniform elongation (uEL) were measured. The tensile test was performed by the method prescribed by JIS Z 2241 using a JIS No. 5 tensile test piece. The uniform extension test was performed by a method prescribed in JIS-Z2201 using a JIS No. 5 test piece having a parallel portion length of 50 mm.

The critical bending radius R is such that the direction perpendicular to the rolling direction is the long side direction (the bending ridge line is consistent with the rolling direction), the test piece No. 1 described in JIS Z2204 is made, and then the V bending test is performed in accordance with JIS Z2248. . Bend the sample with only the surface softened part of the single mask to the surface with the surface softened part to bend the outside. The angle between the die and the punch is set to 60 °, and the radius of the punch front end is changed by a unit of 0.1 mm to perform a bending test, and the radius of the tip of the punch capable of bending without causing cracks is determined as the critical bending radius R. For steel grades with a strength of 780 MPa or more and less than 1180 MPa, those with a critical bending radius R greater than 1.0 mm are considered to have poor bendability (symbol x), those with a strength of 1.0 mm or less are considered to have good bendability (symbol ○), and those with a strength of 0.8 mm or less Excellent bendability (◎). For steel types with a strength of 1180 MPa or more, those with a critical bending radius R greater than 2.0 mm were evaluated as poor bendability (symbol x), those with a diameter of 2.0 mm or less were evaluated as good bendability (symbol ○), and those below 1.5 mm were evaluated as Excellent bendability (◎).

For the obtained steel sheet, the chemical composition at the center position in the plate thickness direction and the chemical composition at the center position in the plate thickness direction of the surface softening part as defined above were actually measured, and then compared with the steel sheet for the surface softening part shown in Table 1 and The chemical composition of the base steel plate is almost the same.

The average Mn concentration is measured at the center of the plate thickness of the tissue cross section and at the center of the plate thickness direction of the surface softening section along the line perpendicular to the plate thickness direction. The Mn concentration at 20 points is measured by EPMA at 50 μm intervals. The average value was calculated. As a result, the average Mn concentration in the center portion of the plate thickness and the average Mn concentration in the surface layer softened portion were almost the same as those of the base material steel plate and the steel plate for the surface layer softened portion shown in Table 1, respectively.

3. Evaluation results The above evaluation results are shown in Table 3.

[Table 3-1] [Table 3-2]

[Table 4]

The underlined values in Tables 1 to 3 indicate the content, conditions, or mechanical properties outside the preferred range.

The steel plates of the examples in Tables 2 and 3 have the following characteristics of steel plates with a high Mn concentration, and have excellent bendability. The foregoing characteristics are: the average Mn concentration at the center of the plate thickness is greater than 4.0% by mass and less than 10.0% by mass; The softened part has a thickness of 0.1% to 30% of the steel sheet, and the average Mn concentration is 2.5% or less; the recrystallization rate of the aforementioned softened part of the surface layer is 90% or more; and, it has excellent TS × uEL balance.

On the other hand, in the materials No. 2, 3, 6, 16, 18, 22, 24, 46, 49, 50, and 52 of Table 2 and Table 3, the recrystallization rate of the surface softening portion exceeds the present invention. Outside the specified range, excellent bendability cannot be obtained.

The average Mn concentration in the center of the plate thickness of the tested material No. 14 was low, and an excellent TS × uEL balance could not be obtained.

The annealing temperature of the tested material No. 32 was high, and the excellent TS × uEL balance could not be obtained.

The average Mn concentration in the softened portion of the surface layer of the test material No. 33 was high, and excellent bendability was not obtained.

The thickness of the softened portion of the surface layer of the tested material No. 38 was small, and excellent bendability was not obtained.

The thickness of the surface softened portion of the tested material No. 39 is large, and the strength is low.

The tested materials No. 46, 49, 50, and 52, although they are multilayer steel plates produced by cold-rolling with a hot-rolled plate as the substrate, have a low recrystallization rate of the stacked layer (surface layer). Can obtain excellent bendability.

The tested materials Nos. 47, 48, and 51, although they are multilayer steel plates made of cold-rolled plates as substrates and produced by cold spraying methods, have pores in the build-up layer (surface layer), and have not obtained good results. Flexibility. In addition, the average crystal grain size of the recrystallized structure in the softened portion of the surface layer is large, and excellent bendability cannot be obtained.

Industrial Applicability According to the present invention, a high-strength steel sheet having excellent bending workability and a high Mn content suitable for automotive blanks can be produced with high production efficiency, and has many industrial advantages.

Claims (16)

A high-strength steel plate includes a center portion of a plate thickness and a softened portion of one or both surfaces formed at the center portion of the plate thickness. The high-strength steel plate is characterized in that the average Mn concentration in the center portion of the plate thickness is greater than 4.0 mass % And less than 10.0% by mass, each surface layer softened portion has a thickness of 0.1% to 30% of the plate thickness, the average Mn concentration of the surface layer softened portion is 2.5% by mass or less, and the surface layer softened portion has a recrystallization rate of 90% or more, The average crystal grain size of the recrystallized structure in the softened portion of the surface layer is 0.1 μm or more and 40 μm or less. For example, the high-strength steel sheet according to claim 1, wherein the center of the aforementioned plate thickness is contained in mass%: C: more than 0.05% and less than 0.80%, Si: 0.001% and more and less than 3.50%, and Mn: more than 4.0% and less than 10.0% , P: 0.10% or less, S: 0.010% or less, sol.Al: 0.001% or more and less than 3.00%, and N: less than 0.050%, and the remainder is composed of iron and unavoidable impurities. The high-strength steel sheet according to claim 2, wherein the center of the aforementioned plate thickness includes at least one element selected from the group consisting of: Cr: 0.01% or more and 2.00% or less, Mo: 0.01% Above: 2.00%, Cu: 0.01% or more and 2.00%, and Ni: 0.01% or more and 2.00% or less. The high-strength steel sheet according to claim 2 or 3, wherein the center of the aforementioned plate thickness includes at least one element selected from the group consisting of: Ti: 0.005% or more and 0.30% or less, Nb: 0.005% or more and 0.30% or less, V: 0.005% or more and 0.30% or less, and W: 0.005% or more and 0.30% or less. The high-strength steel sheet according to claim 2 or 3, wherein the center of the aforementioned plate thickness includes at least one element selected from the group consisting of: B: 0.0001% or more and 0.010% or less, Ca: 0.0001% or more and 0.010% or less, Mg: 0.0001% or more and 0.010% or less, Zr: 0.0001% or more and 0.010% or less, and REM: 0.0001% or more and 0.010% or less. The high-strength steel sheet according to claim 2 or 3, wherein the center of the aforementioned plate thickness includes at least one element selected from the group consisting of: Sb: 0.0005% or more and 0.050% or less, Sn: 0.0005% or more and 0.050% or less, and Bi: 0.0005% or more and 0.050% or less. For example, the high-strength steel sheet of claim 2 or 3, wherein the amount of C in the softened portion of the surface layer is 0.9 times or less the amount of C in the center portion of the plate thickness. The high-strength steel sheet according to claim 3, wherein the sum of the Cr amount and the Mo amount in the softened portion of the surface layer is 0.9 times or less the sum of the Cr amount and Mo amount in the center portion of the sheet thickness. The high-strength steel sheet according to claim 3, wherein the sum of the amount of Cu and the amount of Ni in the softened portion of the surface layer is 0.9 times or less the sum of the amount of Cu and the amount of Ni in the center portion of the sheet thickness. For example, the high-strength steel sheet according to claim 4, wherein the sum of the amount of Ti and the amount of Nb in the softened portion of the surface layer is 0.9 times or less the sum of the amount of Ti and the amount of Nb in the center portion of the sheet thickness. For example, the high-strength steel sheet according to claim 4, wherein the sum of the amount of V and W in the softened portion of the surface layer is 0.9 times or less the sum of the amount of V and W in the center portion of the sheet thickness. The high-strength steel sheet according to claim 5, wherein the amount of B in the softened portion of the surface layer is 0.9 times or less the amount of B in the center portion of the plate thickness. The high-strength steel sheet according to any one of claims 1 to 3, further comprising a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electro-galvanized layer on the surface of the aforementioned surface layer softened portion. A method for manufacturing a high-strength steel sheet is a method for manufacturing a high-steel steel sheet according to any one of claims 1 to 13. The method for manufacturing a high-strength steel sheet is characterized in that it includes the following steps: One or both sides of the steel sheet are laminated to form the steel sheet for the surface softening part of the surface softening part to form a multilayer steel sheet; the multilayer steel sheet is heated to 1080 ° C to 1300 ° C, and the finishing rolling start temperature is 800 ° C Hot rolling is performed under the conditions above and below 1000 ° C; cooling the hot-rolled multilayer steel plate to 500 ° C or higher and 700 ° C or less within 2 seconds after the finish rolling is completed; cooling the multilayer steel plate to After the temperature of 500 ° C or more and 700 ° C or less, the temperature is maintained for 3 seconds or more; the multilayer steel plate is maintained at a temperature of 500 ° C or more and 700 ° C or less for 3 seconds or more, and then pickled at 20% or more and 70% or less Cold rolling is performed at a reduction ratio; and the multilayer steel sheet after the cold rolling is maintained at a temperature of 600 ° C. or higher and 750 ° C. or lower for 5 seconds or more, followed by cooling. For example, the method for manufacturing a high-strength steel sheet according to claim 14 uses a coiling temperature of 600 ° C. or lower to wind the multilayer steel sheet that is maintained at a temperature of 500 ° C. or higher and 700 ° C. or lower for 3 seconds or longer. According to the method for manufacturing a high-strength steel sheet according to claim 15, before the cold rolling, the multi-layer steel sheet after the coiling is maintained at a temperature of 300 ° C or higher and 550 ° C or lower and tempered.